Nonframework cations

Ion Exchange. The exchange behavior of nonframework cations in zeoHtes, eg, selectivity and degree of exchange, depends on the nature of... 

Another way to modify zeolite properties is to change the nature of the nonframework cations. Some examples of how cation exchange can affect properties are noted in Section 2.6. For catalytic applications it is often desirable to create active acidic sites. This can be done by exchanging the cations with NHJ and then calcining the zeolite, removing NH3 and leaving behind protons attached to framework oxygen atoms. 

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

The surface character of the AlPO molecular sieves differs from that of the silica molecular sieves even though both framework types are neutral with no extra-framework cations. The molecular sieve silicalite is hydrophobic and the AlPO molecular sieves are moderately hydrophilic. Zeolites are hydrophilic due to the interaction of the dipole of the Hz0 molecule with the electrostatic fields of the anionic aluminosilicate framework and the balancing nonframework cations. The hydrophilicity of the AlPOi, materials is apparently due to the difference in electronegativity between Al(1.5) and P(2.1). Neither mechanism is possible with silica molecular sieves. The AlPOi, molecular sieves do exhibit less affinity for HzO than the hydrophilic zeolites such as Type A and Type X. 

The energetics of molecule sorption depend on the guest molecule or atom, the specific cations present in the framework, and the Si Al ratio. Frameworks with low levels of aluminum substitution and few nonframework cations have relatively low affinities for polar molecules. 

Figure 2.46 Framework structure of ECR-34 viewed down the c direction, showing the 18-ring channel. Framework oxygen atoms and nonframework cations are omitted for clarity. Reproduced with permission from [116], Copyright (2003) American Chemical Society...

Bonding between the zeolite lattice and organic molecules adsorbed in the zeolite cavities is best described as a van der Waals-type weak interaction, when interaction solely occurs with the silica zeolite wall. The bonds should be considered to be hydrogen bonds if interaction occurs with the acidic protons bonded to the zeolite framework. The latter type of bond has been discussed in section 4.3. Interaction with the nonframework cations belongs to a category of bonding in between these two extremes. 

Siting, Oxidation State, Coordination Sphere of Framework and Nonframework Cations... 

The properties of zeolites are decisively dependent on their cations, either on the framework cations introduced by replacement of Si or Al in the zeolite lattice or on the charge-compensating nonframework cations inserted by ion exchange in the solid state or from solutions. This opens possibilities for targeted modification of these materials in order to tailor their properties for specific applications. 

The crystallographic sites of nonframework cations in the structures of zeolite A (LTA) and zeolites X and Y (FAU) are indicated in Fig. 22 where their coordination to the framework is shown in the lower part. Their nomenclature, sites per unit cell, site symmetry and location are summarized in Table 3. Note that the SI position in LTA is equivalent to site SII in FAU and will be further designated as SIA. 

Table 3. Nomenclature, number per imit cell (u.c.), symmetry and location of nonframework cation sites in zeolites LTA and FAU ...

Fig. 36. Wavelength shift AA. of the electronic band of aniline (1), p-phenylenediamine (2), pyridine (3) and nitrobenzene (4) adsorbed on zeolites UX, NaX, KX, RbX and CsX as a function of the electrostatic potential U=e/r (A) of the respective nonframework cation. Reprinted with permission from [106], Copyright 1971 American Chemical Society...

For inorganic systems, it has been more common to specify pairwise interaction parameters individually (see, e.g.. Ref. 86). This is a consequence of the historical emphasis on the simulation of a limited number of compounds within a particular study. However one notable exception is provided by the ionic potential parameters derived by Vessal (see Table 12 in the later section Zeolites and Microporous Materials). In his approach, combining rules were a design constraint in the derivation of the parameter set as a whole. The resulting force field is effective for both the structural and energetic description of a wide range of materials and, in particular, the incorporation of nonframework cations in microporous materials, which can prove an exacting test of more locally derived and less transferable force fields. 

Apart from plain hydrogen bonding, three main modes of adsorption of basic probe molecules have been identified by adsorption calorimetry, namely adsorption on surface silanol groups, adsorption on cations, and adsorption on acid sites that are associated with protons occupying nonframework cationic positions. Adsorption may also occur on Al-containing non-framework species which are not cationic in nature, e.g., oxidic species. 

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